Endothermic & Exothermic Reactions (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Heat exchange in reactions

• Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons
  • This is done by chemical bonding 
• This process involves the transfer of thermal energy into and out of reaction mixtures
• The terms used to describe this are:
  • System: the reacting chemicals 
  • Surroundings: anything other than the chemicals reacting
• The energy within the system comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy

Exothermic reactions

• In exothermic reactions, thermal energy is transferred from the chemical energy store of the chemical system to the surroundings
  • The energy of the system decreases, which means that the energy change is negative
  • The temperature of the surroundings increases because thermal energy is given out / released
• The overall transfer is from the system to the surroundings

Diagram showing the transfer of heat energy outwards from an exothermic reaction

• Typical examples of exothermic reactions include:
  • Combustion
  • Oxidation
  • Neutralisation 
• Hand warmers used in the wintertime are based on the release of heat from an exothermic reaction
• Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers

Endothermic reactions

• In endothermic reactions, thermal energy is transferred from the surroundings system to the system surroundings
  • The energy of the system increases, which means that the energy change is positive
  • The temperature of the surroundings decreases because thermal energy is taken in / absorbed
• The overall transfer is from the surroundings to the system

Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction

• Endothermic reactions are less common than exothermic reactions
• Typical examples of endothermic reactions include:
  • Electrolysis
  • Thermal decomposition
  • The first stages of photosynthesis 
• Cold packs for sports injuries are based on endothermic reactions, designed to take heat away from a recently injured area to prevent swelling